1. Field of the Invention
This invention pertains generally to thermionic cathodes and more particularly, to reservoir-type dispenser cathodes that find particular advantageous application in devices such as cathode ray tubes requiring very high current density, that is, current densities greater than 10 amps per square centimeter of cathode surface area. The cathode of the present invention also finds advantageous application where current density requirements are less than 10 amps.
2. Prior Art
The most relevant prior art known to the applicant is U.S. Pat. No. 4,165,473 which discloses an improved cathode invented by the inventor of the present invention and which is assigned to Varian Associates, Inc. of Palo Alto, Calif. That patent discloses a dispenser cathode comprising a porous metal matrix consisting of a compacted mixture of tungsten and iridium particles impregated with a molten barium aluminate and other alkaline earth oxides which may be added to the matrix. The cathode structure disclosed in U.S. Pat. No. 4,164,473 is apparently primarily intended for use in microwave electron tubes designed for continuous wave operation such as a Klystron amplifier.
The prior art section of that patent adequately describes the previous attempts to provide cathodes capable of generating high current densities and indicates that generally the prior art limit of current density from such prior art attempts was about 3 amperes per square centimeter of cathode surface area. Furthermore, that patent reveals a structure which is capable of generating at least 10 amperes per square centimeter of cathode surface area thus providing a significant increase in power particularly at very high frequencies for use in microwave devices.
The dispenser cathode of the aforementioned patent was primarily intended for specialized microwave tubes which are generally very costly. Therefore, the high cost of manufacturing such cathodes was not at the time considered a major disadvantage. Dispenser cathodes costing as much as ten to twenty dollars to manufacture were not considered too expensive for their application in microwave tubes costing as much as thousands of dollars. On the other hand, thermionic cathodes designed for use in cathode ray tubes such as those used in computer terminals and displays and in certain TV monitors, have always been considered very cost sensitive because of the high volume and competitive nature of the ultimate product into which those cathodes are installed. Consequently, cathodes used in the prior art for such cost sensitive applications in cathode ray tubes have generally been of the type comprising an insulator semiconductor oxide cathode combination which is not capable of current densities greater than about 1 amp per square centimeter of cathode surface area, but which was still adequate for the relatively low current density applications of such prior art CRT devices.
Unfortunately, significant improvements in the computer art specifically related to display applications as well as other advances in cathode ray tube applications, have created a demand for a cathode for use in cathode ray tubes which is capable of achieving the high current densities of 10 amps per square centimeter or greater thereby making the dispenser-type cathode a highly desirable electron beam source for more recent cathode ray tube applications. However, the manufacturing costs of such dispenser cathodes continues to be about an order of magnitude higher than that which would be feasible in the highly competitive, cost sensitive cathode ray tube industry.
Thus there is now a need for a dispenser-type cathode which is capable of the aforementioned higher current densities but which may be manufactured for approximately 1/10 of the manufacturing costs of previously known high current density dispenser cathodes. Thus there are, in effect, two types of prior art to which the present invention may be compared. On the one hand there is the costly dispenser cathode prior art which is substantially unsuitable for application in cathode ray tubes because of the cost sensitivity of the ultimate product. On the other hand, there are the conventional cathodes that have previously found application in cathode ray tubes because of their relatively low cost but which are incapable of providing the high current densities that the more demanding applications require of today's cathode ray tubes.
The latter prior art, that is, prior art cathodes that have conventionally been used in cathode ray tubes, employ a nickel substrate with an impurity of magnesium or silicon as activators and which is coated with barium oxide, calcium oxide or strontium oxide applied as carbonates and which decompose to oxide during manufacture. Unfortunately, electron emission from such conventional cathode ray tube cathodes is far too limited for today's applications because the electron emission is induced from a semiconductor material and in order to increase the current density such materials require an extremely high voltage. Such high voltages applied for longer than a short pusle can cause arcing which is destructive to the cathode as a result of the charging effect of the material. The limit of current density therefore has usually been less than one amp per square centimeter for cathodes in CRT applications.
Previous attempts to substitute a metal cathode for the semiconductor cathode of the CRT art, such as the attempt described in the disclosure of U.S. Pat. No. 4,165,473, have been limited to metals that can survive a hydrogen atmosphere used during the impregnation step such as where tungsten is impregnated with barium aluminate or barium calcium aluminate or other earth metal additives.